Cells from nonhuman species are often not permissive to HIV replication due to an absence of factors required by the virus or due to the presence of factors that interfere with virus replication. We are screening for positive and negative regulators of HIV infection to better understand interactions between HIV and the host cell during virus replication. These studies may provide new drug targets for antiviral therapy and identify cell factors that might be induced to interfere with HIV replication. Identification of HIV host cell factors will also enable the improvement of cell culture and animal models to study HIV transmission and replication. In the course of these studies, we have discovered a postentry block to HIV replication unique to mouse T cells. Mouse fibroblasts support the early steps in HIV replication including reverse transcription, nuclear import, and integration into host cell DNA; however, in murine T cells there is a defect in the apparent nuclear import of the viral genome. This restriction to HIV replication is epigenetic. Murine fibroblasts are 100-fold more permissive for the early steps of HIV replication compared with T cells from the same mouse strain. Recent experiments indicate that the block is recessive, suggesting that a factor necessary for HIV replication is diminished in the mouse T cells. We are using complementation and cDNA screening approaches to identify the factor(s) required. These studies will provide insights into the early replication requirements of HIV, identify targets for possible antiviral therapy, and aid in the development of genetically altered mice that are susceptible to HIV infection.Using a genetic screen, we have identified a cDNA from a mouse thymus library that has potent antiviral activity against HIV-1, HIV-2, and SIV, but no activity against MLV. Our studies indicate that the antiviral protein is a truncation of an SR-related, pre-mRNA-associated factor. Ectopic expression of the truncated SR-protein restricts HIV infection in human or mouse cells by two orders of magnitude. The accumulation of 2-LTR circles is markedly reduced in the presence of the truncated SR-protein. We plan to further investigate the mechanism of the truncated SR-protein restriction of HIV. We also plan to determine whether the truncated SR-protein interferes with a protein that normally contributes to HIV replication. These studies attempt to identify cell factors that interact with the HIV reverse transcription complex (RTC) as well as provide insights about the transition from viral RTC to preintegration complex (PIC).